[test] Add tests for mutating a global

The error is thrown correctly but the error message is incorrect.

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.mutate-property-from-global.expect.md

@@ -0,0 +1,21 @@
+
+## Input
+
+```javascript
+let wat = {};
+
+function Foo() {
+  delete wat.foo;
+  return wat;
+}
+
+```
+
+
+## Error
+
+```
+[ReactForget] InvalidReact: This mutates a variable after it was passed to React, which means that React cannot observe changes to it (4:4)
+```
+          
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.mutate-property-from-global.js

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+let wat = {};
+
+function Foo() {
+  delete wat.foo;
+  return wat;
+}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.store-property-in-global.expect.md

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+
+## Input
+
+```javascript
+let wat = {};
+
+function Foo() {
+  wat.test = 1;
+  return wat;
+}
+
+```
+
+
+## Error
+
+```
+[ReactForget] InvalidReact: This mutates a variable after it was passed to React, which means that React cannot observe changes to it (4:4)
+```
+          
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.store-property-in-global.js

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+let wat = {};
+
+function Foo() {
+  wat.test = 1;
+  return wat;
+}